Earth boring bits include rotary cone rock bits that are employed for drilling wells, blast holes, or the like in subterranean formations for oil, gas, geothermal steam, minerals, and the like. Such drill bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and/or the bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled. As such a rock bit is used for drilling deep wells where tough formations, high pressures and temperatures are encountered.
When a drill bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The amount of time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the service life of a drill bit in a rock formation. Prolonging the time of drilling minimizes the time lost in "round tripping" the drill string for replacing the bits. Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation.
One cause of rock bit failure is due to severe wear that occurs on journal bearings on which the cutter cones are mounted. These bearings can be friction or roller-type bearings and can be subject to high drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. The journal bearings are lubricated with grease adapted to such severe conditions. The grease is retained within the rock bit, to lubricate the journal bearings, by a seal. The seal is typically in the form of an annular ring and includes a dynamic seal surface, that is placed in rotating contact against a journal surface, and a static seal surface, that is placed in contact against a stationary or relatively stationary cone surface. The seal must endure a range of different temperature and pressure conditions at the dynamic and static seal surfaces during the operation of the rock bit to prevent the grease from escaping and/or contaminants from entering and, to thereby ensure that the journal bearings remain sufficiently lubricated.
Journal seals used in such earth boring bits are typically provided in the form of a single annular O-ring-type seal that is intended to provide an absolute or positive seal, and that is made entirely from rubber or other elastomeric material. The term "absolute" or "positive" seal is intended to mean that the seal is designed to fit within a complementary seal cavity to provide a leak-tight seal between the journal and cone surfaces, thereby prohibiting both leakage of grease from the bit and ingress or leakage of drilling fluid into the bit. It is desirable that a secondary seal be used in conjunction with a primary journal seal to reduce the amount of drilling debris or the like that can reach the primary seal. Such secondary seals perform the function of excluding passage of debris towards the primary seal.
Journal seals formed from such rubber or elastomeric materials display excellent sealing properties of elasticity and conformity to mating surfaces, they display poor tribiological properties, low wear resistance, a high coefficient of friction, and a low degree of high-temperature endurance and stability during operating conditions. Accordingly, the service life of rock bits equipped with such seals is defined by the limited ability of the elastomeric seal material to withstand the different temperature and pressure conditions at each dynamic and static seal surface.
U.S. Pat. No. 3,778,654 issued to Mandley discloses a multiple hardness O-ring comprising a seal body formed from nitrile rubber, and a hardened exterior skin surrounding the body that is formed by surface curing the exterior surface of the nitrile rubber. Although the patent teaches that the O-ring seal constructed in this manner displays improved hardness and abrasion resistance, the act of hardening the entire outside surface of the seal body causes the seal to lose compressibility and other related properties that are important to the seal's performance at the static seal surface.
U.S. Pat. No. 4,557,609 issued to Moren discloses a drill bit seal having a dynamic and static seal surface formed from different materials. The dynamic seal surface is formed from a relatively low friction material comprising Teflon that is deposited onto a inside diameter surface of the seal. The static seal surface is formed from the same material that is used to form the seal body. The Teflon surfaces acts to improve the wear resistance of the seal at the dynamic seal surface. However, the use of Teflon on the dynamic seal surface only provides a temporary improvement in wear resistance because it eventually wears away to uncover the relatively less wear resistant seal body.
U.S. Pat. No. 5,362,073 issued to Upton et al. discloses a composite rock bit seal comprising a dynamic seal surface, formed from a single type of elastomeric material, and has inner and outer static seal surfaces that are each formed from different elastomeric materials. The elastomeric materials used to form the static seal surface are less wear resistant than the elastomeric materials forming the dynamic seal surface. The materials forming the dynamic and static seal surfaces are bonded together by cross-linking to form the seal body. Although the seal provides a degree of improved wear resistance at the dynamic seal surface, it is still limited to what an elastomer can offer in terms of wear resistance, therefore the dynamic surface geometry will still be the point of failure of the seal.
It is, therefore, desired that a journal seal be constructed in a manner that displays sealing properties that are equal to or better than those of seals formed exclusively from elastomeric materials when used in sealing applications for an earth boring bit. It is also desired that the seal construction display improved tribiological properties, improved wear resistance, a reduced coefficient of friction, and improved high-temperature endurance and stability when compared to conventional journal seals formed exclusively from elastomeric materials.